The present invention relates to the field of integrated circuits, their operation, and manufacture. More specifically, in one embodiment the invention provides improved dynamic random access memories, methods of operating a dynamic random access memories, and methods of making a dynamic random access memories.
In the attempt to increase the number of bits on present DRAMs, elaborate processes are proposed for shrinking device dimensions, while still maintaining high enough capacitance in the storage capacitors so that data can be reliably stored, refreshed, and read. For a given bit line capacitance, parameters of primary importance in a read operation are the proportional capacitance of the storage capacitor and the voltage difference between the capacitor and the bit line. The "primary" signal in a DRAM, which is the signal on the floating bit lines before enabling the amplifier, must be larger than the deviation in the switching point between the two nodes of the amplifier.
Deviation of the switching points in the amplifier arises due to the variation in the transistor parameters of the sense amplifier. In a typical DRAM, this variation is often at least 100 mV. Other factors, such as supply variations and coupling from crossing signals may require even larger primary signals. To satisfy this requirement, a cell capacitance of 30 fF is typically required in today's DRAM devices.
While meeting with substantial success, such DRAMs have also met with certain limitations. For example, as the size of DRAMs has increased due to reductions in feature size, difficulties have been encountered in keeping the primary signal above the needed level. This has, in turn, limited the number of bits that can be placed on a bit line and, therefore, the achievable number of bits on a single DRAM substrate. Other difficulties include decreased yield and the like.
From the above it is seen that an improved dynamic random access memory is needed, along with improved methods of operating such memories and improved methods of making such memories.